1. Field of the Invention
The present invention relates to a process for the preparation of high quality coke. More particularly, the present invention relates to a process for preparing coke having improved physical properties, particularly improved apparent density and thermal expansion characteristics for production of graphite electrodes, by calcining green coke obtained by the delayed coking process under novel calcination conditions.
2. Description of the Prior Art
The so-called coking process for cracking heavy oil to obtain light products, i.e., gas, cracked naphtha and cracked gas oil, and heavy products, i.e., coke formed by the condensation reaction, is known is the art. As one modification of this coking process, there is known a so-called delayed coking process in which feed oil is heated in a heating furnace and coke is formed in a coking drum.
It is also known that green coke prepared by the delayed coking process can serve as a carbonaceous material for production of graphite electrodes through a calcination step.
This calcination is performed for removing volatile matters (as measured according to ASTM D-271) from green coke so as to obtain coke suitable for production of electrodes. Ordinarily, this calcination comprises a heat treatment at a temperature about 1200.degree. to 1500.degree. C. using a rotary kiln, a retort, a shaft kiln or the like.
Since the conditions under which electrodes are applied have recently become more severe, coke for production of artificial graphite electrodes is required to have special properties, especially a low coefficient of thermal expansion and a high apparent density.
For example, calcined coke prepared by using green coke obtained by the conventional delayed coking process an calcining it at the above-mentioned conventional calcination temperature has, in general, an apparent density of about 1.30 g/cm.sup.3 or lower (as measured by the water coating method described hereinafter) and thermal expansion characteristics such that the coefficient of thermal expansion of an article (shaped from the calcined coke) baked at 1000.degree. C. is about 1.9 .times. 10.sup.-6 /.degree. C. or higher and the coefficient of thermal expansion of an article (shaped from the calcined coke) graphitized at 2600.degree. C. is about 1.2 .times. 10.sup.-6 /.degree. C. or higher (each coefficient of thermal expansion being a coefficient of linear thermal expansion as measured by the measurement method described hereinafter). These calcined coke products are used for production of graphite electrodes of the common grade. However, for the production of electrodes to be used under severe conditions, for example, high power electrodes, the starting coke is required to have an apparent density of about 1.35 g/cm.sup.3 or higher as measured by the above-mentioned method and thermal expansion characteristics such that the linear thermal expansion coefficient of an article baked at 1000.degree. C. is about 1.5 .times. 10.sup.-6 /.degree. C. or lower as measured by the above method and that of an article graphitized at 2600.degree. C. is about 1.0 .times. 10.sup.-6 /.degree. C. or lower as measured by the above method.
As regards the measurement of the coefficient of thermal expansion (C.T.E.), no standards are fixed in the art on the preparation of test pieces, the graphitizing treatment temperature and the measurement methods, and the thermal expansion characteristics are evalulated by the values determined by various different methods. The specific values of the thermal expansion coefficient mentioned above are those determined by the test method voluntarily adopted by us, which will be detailed hereinafter.
Various methods have heretofore been proposed for obtaining high quality coke meeting the above requirements. Most of these proposals are mainly directed to improvements in the process for preparing green coke. For example, there are known a process in which a cracked residuum or cracked tar free of components capable of readily forming an insoluble phase, which is obtained by catalytic cracking or thermal cracking, is used as the raw material and it is formed into green coke according to the known delayed coking process, and a process in which a mixture of a cracked residuum or cracked tar as mentioned above with 10 to 30% of a straight run residuum is used as the feed oil and it is formed into green coke according to the known delayed coking process (see U.S. Pat. No. 2,775,549 and U.S. Pat. No. 2,922,755). There is also known a process in which green coke obtained by subjecting a starting mixed oil comprising 85 to 30% of a straight run residuum and 15 to 70% of a catalytically cracked residuum to delayed coking while recycling all liquid products formed by the coking reaction and having a boiling point higher than 210.degree. C. as measured under atmospheric pressure as the recycle oil is calcined at the above-mentioned calcination temperature (see Japanese Patent Application No. 144687/73).
Improved cokes having a higher quality than coke obtained by calcining green coke from a straight run residuum are obtained according to these known methods. However, each of these previous proposals puts stress on improvements in the starting oil or the delayed coking process, namely the steps for obtaining green coke. In other words, conventional improvements of properties of calcined coke are attained by changing conditions for preparing green coke rather than calcination conditions.
This invention provides a novel process for obtaining high quality coke by improving the calcination step in contrast with the conventional processes.